It is conventional to place pressure responsive switches in communication with automotive hydraulic fluid systems, for example, to provide signals to the powertrain control module (PCM) responsive to the power steering demand so that engine speed can be increased when required to prevent stalling.
A prior art switch system used for this purpose is mounted in a power steering fluid pump and includes a piston slidably disposed in a port which extends from the high pressure side of the pump. A stationary electrical contact is disposed in alignment with the piston in a switch chamber and a second electrical contact is mounted on the end of the piston adjacent to the stationary electrical contact. The piston is normally spring biased away from the stationary electrical contact and is adapted to move into engagement with the stationary contact when the pressure of the fluid increases to a certain level. The fluid which communicates with the piston also is received in the switch chamber so that the contacts move into and out of engagement submersed in the fluid.
This prior art switch system has several limitations including a problem with dirt and contaminants in the fluid which, over time, tend to get between the contacts and interfere with the electrical switching function. Another problem relates to the tendency of the switching level calibration to shift over the life of the device due to friction of the piston in the port. Yet another problem is the low level of contact force which exists at pressures close to the actuation pressure.
U.S. Pat. No. 5,278,367, assigned to the assignee of the present invention, discloses a switch system which overcomes the above noted limitations. In the patent, a piston is shown slidably received in a port formed in the power steering fluid pump. The piston extends into a switch body and has an end disposed in the switch chamber adjacent a switch assembly. The switch assembly comprises a current carrying snap acting disc movable between opposed concave/convex dished configurations. The disc is mounted on an annular disc seat and a stationary electrical contact is mounted in alignment with the center of the disc and located at a selected location within the movement of the disc with the disc providing a selected switching point and hysteresis. A flexible diaphragm disposed between the disc and the piston and sealed with a resilient o-ring between the diaphragm and the switch body prevents the hydraulic fluid from entering the switch chamber.
Although the switch system made in accordance with the patent is very effective, it requires the provision of a precision machined port in the fluid pump housing to receive the piston, as well as an access port to return bleed-by fluid to the low pressure side of the pump which adds to the expense of the pump assembly and makes it unsuitable for certain low cost applications and/or applications where, for one reason or another, it is preferred not to locate the switch in the pump housing.
Another limitation of the switch system of the referenced patent is that the longevity of the snap acting disc member in some applications is adversely affected due to high bending stresses imparted to the disc member as a result of the high pressure fluid pump systems.
It is an object of the present invention to provide pressure responsive electrical switch apparatus which can, in effect, be plugged into the hydraulic system at any convenient location. Another object is the provision of such a switch which has improved longevity and is useful with relatively high temperatures and high vibration levels, which is relatively inexpensive yet rugged and reliable.
Briefly, in accordance with the invention, a high pressure fluid responsive electric switch comprises a housing member formed of steel or other suitable material capable of withstanding high fluid pressure. The housing member has a first threaded end projecting from a central portion with a port or orifice formed through the threaded end extending into a recess formed in the central portion. A shelf is formed around the recess and a cavity is defined by sidewalls extending downwardly from the shelf. An annular support member of steel or the like is received in the cavity capturing a flexible membrane formed of polyimide or the like between the annular support and the shelf forming a fluid chamber in the housing member. The annular support is in turn captured in the cavity by an eyelet formed of an outer ring of steel or other suitable material capable of withstanding the pressures without changing its dimensional configuration, and a terminal post separated from the outer ring by suitable electrically insulating material, such as plastic or glass, in a known manner. The distal free end of the sidewall is deformed inwardly over the ring portion of the eyelet to securely mount the eyelet to the housing. The ring, at its upper side is formed with a disc seating surface spaced along the longitudinal axis of the ring above a distal end of the terminal post and insulating material. A snap acting disc is disposed on the seating surface and is adapted to snap into and out of engagement with a stationary contact on the distal end of the terminal post. A pressure/force converter in the form of a piston element is received in the central opening of the annular support member and has a first end in engagement with the membrane and an opposite second end in engagement with the snap acting disc member. According to a feature of the invention the second end of the piston is formed with a circular force transfer rib having a diameter selected to amplify to a selected level the pressure required to cause the disc to snap from an upwardly convex configuration out of engagement with the stationary contact. According to another feature of the invention, the stationary contact is also formed of a generally circular rib adapted to engage the disc, when it snaps to its opposite configuration, at a location spaced radially outwardly from the center of the disc to reduce the moment arm on the disc between the force converter rib and the location of engagement with the stationary contact. According to a feature of a modified embodiment of the invention, the surface area of contact engagement is reduced by forming the circular rib of the stationary contact into segments, i.e., discontinuous such as a castellated rib.
These and other advantages and features of the invention will become apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.